Introduction

bevy_arbor is AI for Bevy. It gives your agents behavior trees to decide what to do, and perception to tell them what they sense. Environment queries help them pick a place or a target. Smart objects let them find and use things in the world. Navigation connects them to your game’s movement.

This book builds one agent from start to finish: a guard in a 2D side-scroller. Here it is after the first chapter:

use bevy::prelude::*;
use bevy_arbor::prelude::*;
#[derive(Component)] struct Guard;
#[derive(Component, Clone)] struct Patrol { from: f32, to: f32 }
#[derive(Component, Clone)] struct Chase;
fn sees_intruder(seen: &Perceived<Sight>) -> bool {
    seen.active().any(Percept::is_hostile)
}

fn spawn_guard(mut commands: Commands) {
    commands.spawn((
        Guard,
        Sight::new(12.0).with_lose_range(15.0),
        Team(1),
        Facing(Dir3::X),
        Transform::from_xyz(0.0, 0.0, 0.0),
        BehaviorTree::new(behavior_tree! {
            Forever => {
                Select => {
                    when(sees_intruder) => task(Chase),
                    task(Patrol { from: -10.0, to: 10.0 }),
                }
            }
        }),
    ));
}

The guard patrols until it sees the intruder. Then it chases. When the intruder gets away, the guard goes back to its patrol. Later chapters teach it to shoot, reload, take cover, call for help and man a turret.

What you get

• Reactive behavior trees. A branch switches the moment its condition changes. What was running is canceled.
• Trees that sleep. A tree is ticked only when something it waits on changes. An idle tree costs almost nothing.
• Tasks as entities. Each task the tree starts is an entity. Your own systems do the work.
• Perception. Sight, hearing, touch, and allies sharing what they know. It works in 2D and 3D, and you can add senses of your own.
• Environment queries. Score places or things around an agent and pick the best one, such as cover or a target.
• Smart objects. Things in the world with slots an agent can claim and use, such as a turret with a gunner’s seat.
• Navigation, your way. A MoveTo task and path costs for queries, over whatever pathfinding and movement your game has.
• No physics required. Line of sight is a system you can replace. With avian, line of sight and touch come ready-made.

The crates

Most projects only depend on bevy_arbor. It re-exports the other crates behind feature flags.

This book is the guide. The API reference is on docs.rs.